Ultimately cuts down long grass (6)
I believe the answer is:
snitch
'grass' is the definition.
(both mean to inform on or betray)
'ultimately cuts down long' is the wordplay.
'ultimately' indicates one should take the final letters.
'long' becomes 'itch' (to long or itch for something is to desire it).
The final letters of 'cuts down' are 'sn'.
'sn'+'itch'='SNITCH'
(Other definitions for snitch that I've seen before include "Telltale (sl.)" , "Whistleblower (informal)" , "Inform on, tell tales" , "Inform on someone in slangy way" , "Steal; inform (on someone)" .)